So, both Kira and the "crystal" engulfing her are actually the Founder. Which means that when she shot the crystal with her phaser, she was shooting herself. Yet in "Crossover", we see that a single shot from a Bajoran phaser is enough to blow Odo into little itty bitty bits. And later on we will learn that even a low level phaser shot is painful enough that it makes a Founder lose whatever shape they are in. So how can the Founder shoot herself?

Great Moment :

Odo working out that it's not Kira because she told him she loved him - which Kira doesn't. The easiest lie to sell is the one the person wants to believe. Given his feelings for Kira, Odo must have wanted to believe this one very much indeed. Yet he's self aware enough that he doesn't. It's a great moment for his character.

Body Count :

Zero

Factoid :

Actors, producers, and director were all displeased with the look of the 'rock', commenting that no matter what they did it looked unrealistic. Nana Visitor even compared it to being stuck in 'a big old hot fudge sundae, and my head was the cherry on top!' In her case, matters were not helped by the fact that she suffers from claustrophobia.

O'Brien's enjoyment of kayaking is stated as something he has been doing since he served on the Enterprise-D. This is a reference to TNG's "Transfigurations", in which O'Brien's fondness for kayaking is established (along with his tendency to get injured doing it on the holodeck).

This episode reveals the origin of Odo's name - when first discovered by Doctor Mora his sample container was labelled as "Unknown Sample". This was translated into Cardassian as Odo'ital, which literally means "nothing". The scientists then broke that word into two, calling him Odo Ital in the style of a Bajoran name. This was then shortened to Odo.

The plot of this episode was loosely based on the 1964 novel "Sometimes a Great Notion". In the book, a man's leg is trapped underneath a log as the surrounding water level slowly rises.

Salome Jens, who plays the Female Shapeshifter, agreed not to be credited at the beginning of the episode, so as not to spoil the twist at the end.

Aron Eisenberg considers the scene where he confesses his true reasons for wanting to join Starfleet to Commander Sisko to be the best piece of acting he ever did on Deep Space Nine. He enthuses about the chance to do such an emotional scene with Avery Brooks, whom he regards very highly.

Whilst we knew that non-Federation citizens could join Starfleet before, this episode establishes that to do so, a potential Cadet must have a letter of recommendation from a command-grade serving officer.

When Odo threatens the Female Shapeshifter with Kira's phaser, it marks the first time in the series that he ever picks up a weapon.

Plotline

When Kira and Odo chase a Maquis operative to a desolate planet, they quickly lose him in a network of caves. When Kiras foot becomes trapped in a crystal of some sort, they assume that it will be simple to free her - but all their efforts fail, and the crystal gradually engulfs Kira. Is this the end for the love of Odo's life?